1. Field of the Invention
The present invention relates to management of available bit rate (ABR) traffic in asynchronous transfer mode (ATM) networks.
2. Description of the Prior Art
In an ATM network, an ABR type connection is envisaged which negotiates a minimum transmission rate with the network operator when the connection concerned is established but which, in subsequent use of the network, is allowed to exceed its negotiated minimum transmission rate if and when spare capacity in the network arises. Thus, such an ABR connection is not restricted to the minimum transmission rate negotiated at the time the connection was established, and is permitted to transmit at a higher rate than its negotiated transmission rate provided that there is unused capacity in the network.
In the event of congestion, it is envisaged that an ABR connection with a rate exceeding its negotiated minimum transmission rate will first be given a feedback message inviting it to reduce its transmission rate. If an ABR connection does not then reduce its rate, the network will begin to discard traffic in excess of the negotiated minimum transmission rate. When the connection reduces its rate and congestion is alleviated, the connection will be allowed to increase its transmission rate again. Accordingly, ABR connections are at least guaranteed their respective negotiated minimum transmission rates and are in addition allowed to exceed those rates depending on spare capacity in the network and subject to flow control from the network.
Such ABR services are considered desirable in that any connections would prefer to dynamically adjust their transmission rates to the instantaneous available capacity in the network rather than have their ATM cells discarded during congestion as a result of continuing to transmit at higher rates than the network can allow. In other words, ABR services are suitable for connections in which low cell loss is important but some delay (which inevitably results from the need to reduce the transmission rate when congestion occurs) is permissible. ABR services enable connections to use excess capacity in the network, when available, rather than reserve capacity in advance which would involve significant cost outlays.
When a plurality of ABR connections are required in an ATM network, the sources associated respectively with these ABR connections effectively compete for available bandwidth in the network. It is desirable in such circumstances to provide a system for allocating the available bandwidth in a fair manner.